Here's hoping my Windows 7 disc arrives soon, because I've had enough of Vista.
Explorer window: Suddenly, for no reason, this will forget what size it was. It will open up about 640x480 and I will have to resize it.
Start Menu: I've all sorts of problems with this, mainly due to permissions. It wouldn't allow programs to create shortcuts on installation, I couldn't move items to where I wanted etc.
Worst, a few moments ago, the Games folder vanished. Poof! Just gone. After half an hour searching for it, I had to restore from yesterday's backup to get it back...